The silica-rich ZSM-5 zeolite and its homologue, the ZSM-11 zeolite, belong to the crystalline alumino-silicate pentasil family. These zeolites are used in many catalytic reactions of industrial interest such as xylene isomerization, toluene disproportionation, benzene and toluene ethylation, and methanol-to-gasoline conversion among others. Their peculiar catalytic properties are mainly due to their regular framework with a pore size which is intermediate to the large pore sized zeolites (for instance, zeolites X and Y) and the small pore sized zeolites (for instance the A zeolites). The shape selectivity of the pentasil zeolites is the catalytic expression of many factors such as:
(a) the sieving effect, i.e. the capability of the zeolite to admit its pores or to reject reactive molecules having a critical diameter falling within a well defined range;
(b) the (reverse) sieving effect, i.e. the capability of the zeolite to allow product molecules having a certain critical diameter to diffuse out of its pores. Thus, in the case of a product molecule having a diameter exceeding the pore size of the zeolite, this molecule will have to undergo additional cracking into a smaller molecule before diffusing out of the zeolite;
(c) the effect on the reaction intermediates, i.e. the capability of certain active sites to determine the length and geometry of reaction intermediate species.
In the dehydration-cracking of methanol, this third factor is enhanced by using a ZSM-5 zeolite in acid or H-form i.e. one in which nearly all the sodium ions originally present in the zeolite as synthesized, have been exchanged for protons. Thus, according to the scientific literature, the (H-form) ZSM-5 zeolites, when reacted with light alcohols, including particularly methanol and ethanol, give very similar product distributions. These results are in perfect agreement with theoretical studies on the reaction mechanism, which have identified an identical first reaction step leading to olefinic precursors, namely propylene and ethylene.
The acid sites in ZSM-5 zeolites are mainly located at the zeolite channel intersectons and are also responsible for the formation of aromatics in the final products of methanol conversion. Although the "narrowness" of the zeoite channels restricts, as mentioned before, the size of reaction products to those having no more than 11 C-atoms, the acidity of the reactive sites promotes the formation of hydrocarbons having at least 4 carbon atoms from light olefinic precursors such as ethylene and propylene. Therefore, a modification of the acid sites of the ZSM-5 zeolites is warranted if the production of shorter hydrocarbons such as ethylene is desired. By activating the zeolite at high temperature, Lewis acid sites are also formed by dehydroxylation of the zeolite surface.
In any event, it is now also through that the acid character of the acid form of the ZSM-5 zeolites originates in the Bronsted centres created by the tetrahedral aluminium sites.
According to the actual prior art, ethanol conversion over gamma-alumina mostly leads to the corresponding ether and only a small amount of ethylene is normally formed. Higher selectivities to ethylene may be obtained with very acidic aluminas and under certain reaction conditions.
Ethanol reacts over certain oxides such as thorium, chromium or titanium oxides to produce simultaneously ethylene and acetaldehyde, a dehydrogenation product. The decomposition of ethanol over supported thoria catalysts produces almost exclusively ethylene. However, conversion rates are low and an ethanol concentration of at least 90% is necessary to provide a significant yield in ethylene. In fact, if the dehydration of alcohol is performed over the afore mentioned thorium, chromium or titanium oxide catalysts with ethanol in very dilute aqueous solutions (2 to 19 vol % of ethanol in water), the yield in ethylene is low although the selectivity to ethylene in the product hydrocarbons can be very high. This is due to the fierce competition in the absorption onto the catalyst surface, between the molecules of ethanol and water. The detrimental effect of water on the yield in ethylene is due to the stronger adsorption of water on the hydrophilic active sites of the catalyst.
The silica-rich ZSM-5 zeolites, on the other hand, exhibit a large hydrophobic surface which prevents saturation of the catalyst surface by water molecules.
In 1984, a study done by G.A. Aldridge et al. and published in Ind. Eng. Chem. Process Res. Dev. 23, 733-737 (1984) showed that the catalytic conversion of ethanol from fermentation broths to gasoline over a pentasil type zeolite was at the time the most efficient in terms of energy requirement. Such a process comprises two steps:
(a) distillation of the fermentation broth in order to obtain a 60 wt% ethanol aqueous solution (as optimum concentration of ethanol, suitable for the next catalytic step) and then;
(b) catalytic reaction at 300.degree. C. after a compression of the resulting vapors up to 8 atm.
Of course, the major inconvenient of such a technique is the need to perform distillation of dilute ethanol solutions in order to obtain significant yields in the conversion of ethanol to ethylene.
This problem has been overcome in U.S. Pat. No. 4,698,452 which discloses a process for producing light olefins and ethylene in very high yields. This process comprises flowing an aqueous ethanol solution having an ethanol concentration ranging from 2 to 10% by volume through a modified pentasil-type zeolite catalyst at a temperature ranging from 300.degree. to 450.degree. C. The modified catalyst used in U.S. Pat. No. 4,698,452 is a pentasil-type zeolite in which Zn and Mn have been incorporated.
However, the process described in U.S. Pat. No. 4,698,452 is still quite expensive commercially because even though the distillation step has been eliminated, the conversion reaction still has to be performed at a temperature of at least 300.degree. C.
The reaction temperature problem has been partially overcome in U.S. application Ser. No. 102,474 filed Sept. 29, 1987 in the names of R. Le Van Mao and L.H. Dao. This application discloses the use of a steam-treated ZSM-5 zeolite in the production of ethylene from ethanol in water at very low concentrations.
However, as it was the case for the Zn/Mn bearing zeolites of U.S. Pat. No. 4,698,452, the steam-treated ZSM-5 zeolites of U.S. Ser. No. 102,474 again need relatively high temperatures (higher than 275.degree. C.) to completely convert a 2 to 19% aqueous ethanol solution into ethylene. In both cases, such severe reaction temperatures apart from being relatively expensive in terms of large scale commercial operations, favors the thermal decomposition of the leftover glucose and other fermentation residues in the untreated ethanol fermentation broth. The resulting carbon species block the active centers of the catalyst and thereby considerably shorten is active life.
Therefore, a catalyst that could convert very dilute aqueous ethanol solutions to ethylene at low temperatures, thereby preventing thermal conversion of glucose and other fermentation residues and lowering the costs of commercial exploitation associated with high temperatures would be highly desirable.